Ukraine's interior minister says government security forces have launched what he called "an anti-terrorist campaign" in the eastern city of Slovyansk where pro-Russian militants have seized control of official buildings.

Interior Minister Arsen Avakov said on his Facebook page Sunday that gunmen had opened fire on the advancing forces, and that there are dead and wounded on both sides. The report of gunfire and casualties has not been independently confirmed.

On Saturday, Ukraine said attacks by pro-Russian militants in eastern Ukraine are "an act of external aggression" by Russia. Kyiv warned that security officials were preparing to implement "an operational response plan."

In addition to Slovyansk, armed militants with Russian weapons seized police headquarters Saturday in the largely ethnic Russian cities of Donetsk and Kramatorsk.

Around a dozen Berkut, or special forces police, appeared at the Donetsk headquarters, saying they would secure the weapons arsenal in the building.

Witnesses, including Western journalists, say the Kramatorsk facility was captured after a firefight, but there were no reports of casualties.

The takeover of police facilities in Donetsk prompted the city's police chief to resign.

U.S. Secretary of State John Kerry, in a telephone call with Russian Foreign Minister Sergei Lavrov, expressed "strong concern" the attacks on Saturday were orchestrated and synchronized, similar to previous attacks in eastern Ukraine and Crimea. The U.S. State Department says Kerry made clear that if Russia did not take steps to de-escalate in eastern Ukraine and move its troops back from Ukraine's border, there would be additional consequences.

The United States has called on Russia to "cease all efforts" to destabilize Ukraine. A White House National Security Council spokeswoman said Saturday the United States is concerned that Russian separatists - with apparent support from Moscow - are "inciting violence and sabotage" against the Ukrainian state.

In Moscow, Russian state television reports Foreign Minister Lavrov told Kerry Saturday that the crisis is caused by the Kyiv government ignoring "the legitimate needs and interests of the Russian and Russian-speaking population" of the region.

Lavrov also warned that any use of force by Kyiv could undermine diplomatic efforts to resolve the crisis.

Moscow has repeatedly denied any role in Ukraine's unrest, which erupted in full two months ago when then-president Viktor Yanukovych fled the country amid anti-Russian protests in Kyiv.

Despite Moscow's denials, Kyiv and a host of Western governments have cited overwhelming evidence of Russian involvement, including the presence of thousands of Russian troops that infiltrated the Crimean peninsula ahead of last month's secession referendum.

A spokesperson for U.N. Secretary-General Ban Ki-moon said Saturday that the U.N. chief is "deeply concerned" about the deteriorating situation in eastern Ukraine and has appealed to all sides to "adhere to the rule of law and exercise maximum restraint."

Days after that vote, the Russian parliament voted to annex the peninsula, prompting the United States and the European Union to impose economic and travel sanctions on Moscow.

There were other signs of heightened cross-border tensions Saturday, with the Kyiv government saying it is suspending natural gas payments to Moscow. Details of the move were not immediately clear.

Moscow says its neighbor owes $ 2.2 billion in payment arrears. Early this month, the Russian energy giant Gazprom announced two price increases that effectively raise Ukrainian gas costs by about 80 percent. Additionally, Russian President Vladimir Putin has hinted that Moscow may begin demanding energy payments from Kyiv at the time of delivery.

Top diplomats from Russia, the United States, Ukraine and the European Union are set to hold emergency talks on the crisis April 17 in Geneva. White House officials say U.S. Vice President Joe Biden will travel to Kyiv April 22.